1. Field of the Invention
The present invention relates to a method and an apparatus for producing a dope used in a solution casting method.
2. Description Related to the Prior Art
A TAC film formed of cellulose acylate, especially of cellulose triacetate (hereinafter TAC) whose averaged acetylation degree is 57.5% to 62.5% has high strength and inflammability, and is therefore used as a film base of photosensitive materials and the like. Further, as excellent in optical isotropy, the TAC film is used for an optical compensation film, a protective film of a polarizing filter in a liquid crystal display whose market becomes larger recently.
The TAC film is usually produced by a solution casting method, in which the produced film has more excellent physical property (optical properties and the like) than other methods, such as a melt extrusion method. In order to obtain the TAC film in the solution casting method, a polymer solution (hereinafter dope) is produced by dissolving the TAC to a mixture solvent, while a main solvent of the mixture solvent is dichloromethane, methyl acetate or the like. The dope is cast with a casting die onto a smoothly brushed up metallic substrate (or support) to form a casting film. The casting film having a self-supporting property is peeled from the metallic substrate and transferred by a tenter device and a roller drying device with drying.
The dissolubility of the TAC to the solvent is not high. Accordingly, when the dope is produced with use of TAC as the polymer, the production of the dope is hard. Therefore after the TAC is added to the solvent to obtain a dissolving solution (or a rough solution), the dissolving solution is heated and cooled for producing the dope. When the heating and the cooling are made, it is preferable to use a heat exchanger, especially a spiral type heat exchanger. Further, when the dissolution is not made enough, the heating and the cooling are refrained to increase the dissolubility. Further, a temperature gauge may be provided downstream from the heat exchanger to measure the temperature of the solution, and the temperature is regulated by changing the heat transmission conditions of the heat exchanger on the basis of the measured temperature. (For example, Japan Institute of Invention and Innovation (JIII) Journal of Technical Disclosure No.2001-1745).
However, when the dope or the dissolving solution has a temperature distribution, there may be problems in the progress of the dissolution. Further in this case, as the dissolving solution or the dope has the difference of the physical property, especially viscosity, a drift occurs in the dope or the dissolving solution. Thus the uniform heating and cooling becomes hard, and the heating and cooling abilities of the dope producing equipment cannot be enough. Especially, when the heating or cooling is made suddenly, the heat transmission from the transmitting medium to the dissolving solution or the dope cannot be made enough and the temperature distribution easily occurs. Further, when the drift of the dope occurs in the heat exchanger, the drift remains in pipes, and the measured temperature changes depending on position of the measurement. Even though the temperature is incorrectly measured, the adjustment of the heating conditions is made on the basis of the measured values. In this case, the temperature distribution occurs.
Further, it is preferable to keep the pressure to the dope or the dissolving solution higher than the vapor pressure at the highest temperature, while the heating is made. Otherwise, bubbles of the solvent are generated to disturb the dissolution. Further, in this case, the solute, especially the polymer, is solidified in the evaporation of the solvent, and the generated solids materials often occludes the heat exchanger. Further, the solid materials are mixed as foreign materials into the dope.